1. Field of The Invention
The present invention concerns an optical beam scanning device used in image forming apparatus such as laser printers or copying machines.
2. Description of the Related Art
An optical beam scanning device is used, for example, for a laser printer or a digital copying machine. In the optical beam scanning device, a laser beam emitted from a laser light source is deflected by a polygon mirror rotating at a high speed and scans the surface on a light sensitive body.
Since the polygon mirror rotates through the air, when a laser printer or a digital copying machine is used for a long time, oil droplets or paper dusts floating in the apparatus are deposited on the surface of the polygon mirror. In order to avoid such deposition, the polygon mirror is usually sealed together with a driving device in a housing.
In recent years, a processing speed in the laser printer or the digital copying machine has been increased more and the rotational speed of the polygon mirror of the optical beam scanning device is also increased correspondingly. Increase of the rotational speed increase heat generation in a driving device such as a motor for rotationally driving the polygon mirror and a drive circuit for the motor (usually IC is used). Since the polygon mirror and the drive circuit are sealed as described above, the generated heat is less dissipated but confined in the housing to elevate the temperature of them.
Temperature elevation in the drive circuit results in not only fluctuation of electric characteristics of ICs or motors or fluctuation (deterioration) of mechanical characteristics of motor bearings but also fluctuation of optical characteristics of the optical system by thermal deformation.
Heat generated in the motor, the drive circuit and the like is transmitted mainly by way of thermal conduction or convection of internal air to the inner wall surface of a housing that seals them, and further transmitted from the inner wall surface by way of heat conduction to the outer wall surface of the housing and then released from the outer wall surface mainly by convection.
Therefore, in order to improve the heat dissipation, it is required to improve the heat conduction of the housing itself, for example, it is necessary to select a material of high heat conductivity as the material for the housing. Japanese Patent Unexamined Publication Hei 6-75184 discloses a housing a portion of which is made of a metal material with the view point described above.
The invention disclosed in patent publication only utilizes that a metal generally has high heat conductivity. However, in order to improve the heat dissipation, it is necessary to also make the heat conduction path as short as possible, namely, to decrease the wall thickness of the housing as thin as possible since the heat is transmitted mainly along the direction of the thickness of the housing. However, reduction of the wall thickness is 3 mm in aluminum die casting employed usually with a view point of manufacture. The invention disclosed in the above mentioned publication does not pay attention also to these points.
Further, since the polygon mirror rotates at a high speed, vibrations are generated due to slight imbalance in itself or rolling of bearing rolling members. Since the vibrations are transmitted also to the housing, if this frequency is close the natural frequency of the housing, the housing causes resonance and the amplitude of the vibrations of the polygon mirror is also increased. As a result, this leads to a drawback that the axis of an optical beam reflected by the polygon mirror is deflected, or the working life of the bearings is shortened.
The natural frequency of the housing is determined such that it is higher than the frequency of vibrations generated from a rotational portion such as a polygon mirror and apart therefrom as much as possible. However, since the rigidity of the housing is lowered as the wall thickness is reduced, the natural frequency thereof is generally lowered and the frequency generated from the polygon mirror is increased as its rotational speed goes higher, so that the two frequencies are inevitably approach to each other.
The invention disclosed in the publication described above does not pay consideration to such problems of vibrations.
Further, elevation of temperature causes several problems also with respect to a laser light source. The laser light source comprises an LD (laser diode here and hereinafter) and an LD holder, a collimator lens and a collimator lens barrel and, further, a collimator holder for holding the LD holder and the lens barrel and attaching them to an optical housing. Among them, the holder member is a molding product by aluminum die casting or resin molding. Further, a slit controlling the beam diameter is attached to a collimator lens barrel and an LD drive substrate is attached for driving the LD as a unit in some of them.
When the temperature of an atmosphere for the laser light source is elevated, the following phenomena are caused.
(a) [Movement Mainly by Wavelength]
The oscillation wavelength of an LD shifts to a longer wave length area and, as a result, a refractive index of a lens is lowered and a focal point shifts to a positive side.
(b) [Movement by the Change of Distance]
The distance between the LD emission point and the collimator lens is increased by thermal expansion of the collimator holder and, as a result, the focal point shifts to a negative side.
It has been known to offset the movement in a and b to each other so as not to shift the focal point even if the temperature is elevated (Japanese Patent Unexamined Publication Sho 63-7530).
In recent years, since it has been required to reduce the beam diameter on the surface to be scanned along with increase in the recording density, it is necessary, accompanying therewith to enlarge the width of the luminous flux corresponding therewith, tending to make the focal length of the collimator lens longer.
Then, since the depth on the surface to be scanned is made shallow as the beam diameter is reduced, it is necessary to suppress the curvature of image field in order to make the beam diameter uniform within the scanning range and it is necessary to make an f.theta. lens aspherical or free curved surface. Since such a lens shape other than the spherical shape is difficult to be made by glass, it is often formed with a plastic material.
As the focal lens of the collimator lens is increased, the movement (a) described above is increased, and the thermal expansion coefficient (b) has to be increased in order to make a balance with (b).
Further, also in a case of forming the f.theta. lens with a resin lens, the amount of change of the focal point due to the temperature change of the f.theta. lens caused by the change of the wavelength or the refractive index is not negligible and, further, the thermal expansion coefficient (b) has to be increased.
Further, since it is necessary to suppress the elevation of the atmospheric temperature itself in order to maintain the printability, it is necessary to dissipate the heat generated from the light source and the heat transmitted from other members to the light source as rapidly as possible.
Since the error sensitivity for the distance between the LD and the collimator lens is extremely severe upon assembling the later scanning device, it is constituted such that a portion of the collimator holder for receiving the collimator lens barrel is made into a cylindrical or V-shaped block shape, the outer surface of the collimator lens barrel is formed as a cylinder having a rotational axis in common with the optical axis of the collimator lens, and the cylindrical portion is caused to slide in the direction of the optical axis for positioning control.
By the way, since a small lens such as a collimator lens can be formed by glass molding in recent years, configuration into an aspherical or single lens configuration but such a molded lens may sometimes cause non-uniformity during molding although the design performance is improved compared with conventional lenses formed by polishing. If non-uniformity is caused, when collimator lens is rotated around an optical axis as an axis of rotation, the magnitude of a sub-peak changes when the beam shape on the scanning surface is observed. Therefore, it has to be controlled such that the collimator lens is at an optimal angle.
Accordingly, it is desirable that the collimator lens barrel is rotatable by 360.degree. around the optical axis of the collimator lens as the axis of rotation, but the collimator lens barrel includes a boundary of a split mold for injection molding (metal-injection) at the outer cylindrical surface to cause burrs. Therefore, the rotational position can not be controlled over 360.degree. unless machining is applied.